A Digital Television Standard published Sep. 16, 1995 by the ATSC specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of NTSC analog television signals within the United States.
The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. This is done by positioning the pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal that fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal is offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 kHz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690,122.4 Hz from the middle frequency of the television channel.
The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which horizontal scan line rate in an NTSC analog TV signal is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238 * 10.sup.6 symbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
The ATSC standard for digital HDTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV display format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV display format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV display formats other than HDTV display formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) during terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments. The data fields may be considered to be consecutively numbered modulo-2, with each odd-numbered data field and the succeeding even-numbered data field forming a data frame. The frame rate is 20.66 frames per second. Each data segment is of 77.3 microseconds duration. So, with the symbol rate being 10.76 MHz there are 832 symbols per data segment. Each segment of data begins with a line synchronization code group of four symbols having successive values of +S, -S, -S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The initial line of each data field includes a field synchronization code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample pseudo-random noise sequence (or "PN-sequence") followed by three 63-sample PN sequences. The middle ones of the 63-sample PN sequences in the field synchronization codes are transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one's complementary respective to each other.
The data within data lines are trellis coded using twelve interleaved trellis codes, each a 2/3 rate trellis code with one uncoded bit. The interleaved trellis codes are subjected to Reed-Solomon forward error-correction coding, which provides for correction of burst errors arising from noise sources such as a nearby unshielded automobile ignition system. The Reed-Solomon coding results are transmitted as 8-level (3 bits/symbol) one-dimensional-constellation symbol coding for over-the-air transmission, which transmissions are made without symbol precoding separate from the trellis coding procedure. The Reed-Solomon coding results are transmitted as 16-level (4 bits/symbol) one-dimensional-constellation symbol coding for cablecast, which transmissions are made without precoding. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed.
The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 4-bit symbol coding have normalized values of -7, -5, -3, -1, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized value of 1.25. The normalized value of +S is +5, and the normalized value of -S is -5.
In the earlier development of the DVT art it was contemplated that the DTV broadcaster might be called upon to decide whether or not to use a symbol precoder at the transmitter, which symbol precoder would follow the symbol generation circuitry and provide for matched filtering of symbols, when used together with a comb filter in each DTV receiver used before the data-slicer in the symbol decoder circuitry as a symbol post-coder. This decision would have depended upon whether interference from a co-channel NTSC broadcasting station were expected or not. Symbol precoding would not have been used for data line synchronization code groups or during data lines in which data field synchronization data were transmitted. Co-channel interference is reduced at greater distances from the NTSC broadcasting station(s) and is more likely to occur when certain ionospheric conditions obtain, the summertime months during years of high solar activity being notorious for likelihood of co-channel interference. Such interference will not occur if there are no co-channel NTSC broadcasting stations, of course. If there were likelihood of NTSC interference within his area of broadcast coverage, it was presumed that the HDTV broadcaster would use the symbol precoder to facilitate the HDTV signal being more easily separated from NTSC interference; and, accordingly, a comb filter would be employed as symbol post-coder in the DTV receiver to complete matched filtering. If there were no possibility of NTSC interference or there were insubstantial likelihood thereof, in order that flat spectrum noise would be less likely to cause erroneous decisions as to symbol values in the trellis decoder, it was presumed that the DTV broadcaster would discontinue using the symbol precoder; and, accordingly, the symbol post-coder would then be disabled in each DTV receiver.
In automatic symbol post-coder selection circuitry described in previous patents, symbol post-coding is enabled during times when substantial co-channel NTSC interference is detected at the receiver and is otherwise disabled, without regard to whether the broadcaster employs precoding or not. This undesirably would have led to errors in trellis decoding results when the broadcaster did not employ the symbol precoder, but actual co-channel NTSC interference is substantial. These errors would have required that an over-ride of the automatic symbol post-coder selection circuitry be provided for actuation by the human being viewing the DTV receiver. Actual co-channel NTSC interference can be substantial for portions of the reception area for a broadcast without the broadcaster being aware of the condition, owing to freakish skip conditions, owing to cablecast leakage, owing to inadequate intermediate-frequency image suppression in NTSC receivers, owing to magnetic tape used for digital television recording having remnant previous analog television recording, or owing to some other unusual condition.
If there were to be selective employment of symbol precoding by the transmitter, it would be desirable that the use or non-use of symbol preceding by the transmitter be signaled, so that erroneous selection by the automatic post-coder selection circuitry could be avoided. The use or non-use of preceding by the transmitter could be signaled in the data lines used for field sync. Alternatively, the use or non-use of preceding by the transmitter could be signaled in the packet header information for each data packet. Signaling the use or non-use of preceding by the transmitter in the data lines used for field sync is advantageous over signaling in other data lines, in that the signaling is not affected by the use or non-use of post-coding by the digital television receiver.
The current ATSC DTV standard does not specify a symbol precoder operative on all data to be transmitted. The standard presumes that the suppression of co-channel interfering analog TV signal will instead be carried out in the trellis decoding process, after the data-slicing procedures associated with symbol decoding. This procedure avoids the problem of determining whether or not preceding is done at the transmitter. However, co-channel interfering analog TV signal undesirably introduces errors into the data-slicing processes, which places more burden on the error-correction decoding procedures, trellis decoding and Reed-Solomon decoding. These errors will reduce the broadcast coverage area, which may lose revenue for the commercial DTV broadcaster. So, providing for the suppression of co-channel interfering analog TV signal before data-slicing is still desirable, despite symbol preceding at the DTV transmitter not being authorized by the current ATSC DTV standard.
The term "linear combination" in this specification and the claims appended thereto refers generically to addition and to subtraction, whether performed in accordance with a conventional arithmetic or a modular arithmetic. The term "modular combination" in this specification and the claims appended thereto refers to linear combination carried performed in accordance with a modular arithmetic. That type of coding that re-codes a digital symbol stream through differential delay and linear combination of the differentially delayed terms, exemplified by the symbol post-coding used in prior-art HDTV receivers, is defined as "symbol re-coding of first type" in this specification and the claims appended hereto. That type of coding that re-codes a digital symbol stream through its modular combination with delayed result of the modular combination, exemplified by the symbol pre-coding used in prior-art HDTV transmitters, is defined as "symbol re-coding of second type" in this specification and the claims appended hereto.
The problem of co-channel interference from analog television signals can be viewed from the standpoint of being a sometime jamming problem at the receiver, to be solved by adaptive filter circuitry in the receiver. So long as the dynamic range of the system channel is not exceeded, so that the co-channel interference can capture the system channel by destroying signal transmission capability for DTV modulation, the performance of the system can be viewed as a superposition of signals problem. The filter circuitry in the receiver is adapted for selecting the digital signal from the co-channel interference caused by the analog television signals, relying on the pronounced correlation and anti-correlation properties of the analog television signals to reduce their energy sufficiently as to capture the system channel from them.
Insofar as the co-channel interference from analog television signals is concerned, it enters the system channel after the DTV transmitter and before the DTV receiver. The use or non-use of symbol precoding at the DTV transmitter has no effect on the co-channel interference from analog television signals. At the DTV receiver, so long as the co-channel interference is not so large as to overload the receiver front-end and capture the system channel, it is advantageous to precede the data slicing circuitry with a comb filter for reducing the energy of higher-energy spectral components of the co-channel interference, thus to reduce the errors occurring during data slicing. The use of an NTSC-rejection comb filter before data slicing incidentally introduces symbol re-coding of first type.
The data-slicing operation that follows first symbol re-coding in the DTV receiver is a quantizing process that is not destructive of the symbols resulting from the symbol re-coding of first type, insofar as the transmission of data is concerned, since the data quantization levels are designed to match the symbol levels. The quantization discriminates against the co-channel interfering analog TV signal remnants that remain after the filtering associated with symbol re-coding of first type and that are appreciably smaller than steps between symbol code levels, however. This is a species of the capture phenomenon in which phenomenon a stronger signal gains at the expense of a weaker one in a quantizing process
Insofar as the transmission of data is concerned, the digital data symbol stream flows through the full length of the system channel. When symbol re-coding of second type is done as symbol precoding at the DTV transmitter, the additive combination of the differentially delayed data symbol streams is done on a modular basis that does not boost transmitter power or increase average intersymbol distance to help further in overcoming jamming analog TV signal. Instead, the principal mechanism for overcoming jamming analog TV signal is its attenuation vis-a-vis DTV signal, as provided by the comb filtering at the DTV receiver, causing the remnant analog TV signal in the comb filter response to be suppressed by the quantizing effects in the data slicer that immediately follows the comb filter.
The order of performing symbol re-coding procedures of first and second types has no appreciable effect on signal transmission through the system channel under such circumstances, since neither coding scheme destroys signal transmission capability for the symbol stream. The order of performing symbol re-coding procedures of first and second types has no appreciable effect on the capability of the digital receiver to suppress co-channel interfering analog TV signal, presuming symbol re-coding procedures of first and second types are both carried out before data-slicing.
Preferably, the symbol re-coding procedures of first and second types performed by the comb filter for suppressing NTSC co-channel interference and by the comb filter with complementary partial response are carried on continuously without regard to whether the data is synchronizing code or information, the inventor observes. This is because the comb filtering techniques to suppress NTSC artifacts are primarily dependent on the cyclical correlation and anti-correlation properties of the NTSC co-channel interference, which is a continual signal. Because the NTSC co-channel interference is a continual signal of continuous analog nature, the comb filtering techniques to suppress NTSC artifacts should be continuous in their application, even though the results of those techniques may be used only selectively. Although steps have been taken in the trellis coding to boost DTV signal energy in portions of the spectrum relatively free of NTSC signal energy, if data sampling is properly synchronized with symbol rate, the data will be merely re-coded by comb filtering, rather than being destroyed in any significant way. The correlation and anti-correlation of the data symbols is essentially immaterial to the procedure for suppressing the effects of NTSC artifacts on data slicing. The comb filtering that provides independence among the interleaved trellis codes during the trellis decoding procedure after symbol decoding can be separately considered from the comb filtering done during symbol decoding to prevent NTSC artifacts introducing error into data slicing.
U.S. Pat. No. 5,162,900 issued Nov. 10, 1992 to R. W. Citta and entitled "CO-CHANNEL INTERFERENCE FILTER FOR TELEVISION RECEIVER" describes a DTV receiver using a comb filter before the data slicer to reduce artifacts of co-channel interfering NTSC signal, first symbol re-coding being incidental to the use of the comb filter. Symbol re-coding of the second type is interposed between symbol re-coding of the first type and the subsequent data-slicing. The delayed symbols required for the symbol re-coding of the second type are taken from after the data slicer, but symbol re-coding of the second type is completed before data slicing is done.